Addition of an isotonic NaCl solution (e.g., intravenous infusion of 0.9% NaCl: osmolality ≈ 290 mOsm/kg H2O) bSome exceptions do exist. The cerebrospinal fluid is part of the ECF, but its osmolality is slightly higher than that of the ECF elsewhere in the body. Also, regions within the kidney can have osmolalities that are either less than or greater than that of the ECF. However, these volumes are small (≈150 mL) in comparison with the total volume of the ECF (≥12 L). cSerum is derived from clotted blood. Thus serum differs from plasma by the absence of clotting factors. With regard to osmolality and the concentrations of other molecules and ions, the osmolality and concentrations in plasma and serum are virtually identical. dFluids are usually administered intravenously. When electrolyte solutions are infused by this route, equilibration between plasma and interstitial fluid is rapid (i.e., minutes) because of the high permeability of many capillary walls for water and electrolytes. Thus these fluids are essentially added to the entire ECF. eA 0.9% NaCl solution (0.9 g NaCl/100 mL) contains 154 mmol/L of NaCl. Because NaCl does not dissociate completely in solution (i.e., 1.88 Osm/mol), the osmolality of this solution is 290 mOsm/kg H2O, which is very similar to that of normal ECF.
CHAPTER 2 Homeostasis: Volume and Composition of Body Fluid Compartments
Insomeclinicalsituations,itispossibletoobtainamoreaccurateestimateoftheserumosmolality,andthustheosmolalitiesoftheECFandICF,byalsoconsideringtheosmolescontributedbyglucoseandurea,asthesearethenextmostabundantsolutesintheECF(theothercomponentsoftheECFcontributeonlyafewadditionalmilliosmoles).Accordingly,serumosmolalitycanbeestimatedasfollows: 18 28.
Theglucoseandureaconcentrationsareexpressedinunitsofmilligramsperdeciliter(dividingby18forglucoseand2.8for allowsconversionfromtheunitsofmilligramsperdecilitertomillimolesperliterandthustomilliosmolesperkilogramofH2O).Thisestimationofserumosmolalityisespeciallyusefulintreatingpatientswhohaveanelevatedserumglucoseconcentrationsecondarytodiabetesmellitus,andinpatientswithchronicrenalfailure,whoseserumureaconcentrationiselevatedbecauseofreducedrenalexcretion.
,theabilityofasubstancetocausewatertomoveacrosstheplasmamembraneofacelldependsonwhetherthesubstanceitselfcrossesthemembrane.Recall
Eq.1.9 whereΠe= theeffectiveosmoticpressureandσ= thereflectioncoefficientforthesubstance.Formanycells,glucoseandureacrossthecellmembrane.Althoughtheycontributetoserumosmolality,asmeasuredbyalaboratoryosmometerwhereallmoleculesare“effectiveosmoles,”theyareineffectiveosmolesforwatermovementacrossmany,butnotall,cellmembranes.Incontrast,Na+ isan“effectiveosmole”forwatermovementacrosstheplasmamembraneofvirtuallyallcells.
Eq.2.2 givesthebestestimateoftheeffectiveosmolalityoftheserum.
*Theureaconcentrationinplasmaismeasuredasthenitrogenintheureamolecule,orbloodureanitrogen(BUN).